bnxt_en: Return linux standard errors in bnxt_ethtool.c

Currently firmware specific errors are returned directly in flash_device
and reset ethtool hooks. Modify it to return linux standard errors
to userspace when flashing operations fail.

Signed-off-by: Vasundhara Volam <vasundhara-v.volam@broadcom.com>
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>